Presently, there exists no known Standard artifact either in the United States or internationally for certifying or calibrating a multi-axis positioning Coordinate Measuring Machine (CMM) for the application of verifying dimensional accuracy of cooling holes on turbine airfoils.
Current methods rely upon separate three-axis calibration and separate rotational axis calibration. Given the complexity of the configuration of the cooling holes for a turbine airfoil a gold standard is typically employed. The gold standard is an artifact that represents a single version of a part, for example, a turbine airfoil. A turbine airfoil is an appropriate part as the cooling holes pattern of airfoil is typically measured using five-axis positioning Coordinate Measuring Machines. An aft-loaded turbine airfoil, for example, will have certain structural features and a specific pattern of cooling holes for its intended application. In order to be an effective Standard artifact, the gold standard for this particular aft-loaded turbine airfoil must essentially embody the same structural features and specific cooling holes pattern. As a result, the time and craftsmanship required to manufacture a gold standard for each manufactured part measured by a multi-axis positioning Coordinate Measuring Machine is cost prohibitive.
It is also recognized that simply utilizing the actual part, for example, the turbine airfoil, to certify and calibrate the Coordinate Measuring Machine, which inspects the actual part, does not meet basic metrology requirements. Typically, the artifact's accuracy compared to the machine must be within a ratio of 10:1 or 1/10th the accuracy of the machine. For example, if the machine is intended to measure a real part to an accuracy of 1/10,000ths then the machine must be accurate to a 10th of that measurement. Consequently, artifact must be accurate to less than 1/10th of the accuracy of the machine in order to provide a ratio better than 10:1. Again, the workmanship and effort involved to manufacture a Gold Standard having an accuracy to less than 1/10th of the accuracy of the machine for each actual part manufactured and measured by the multi-axis Coordinate Measuring Machine is too expensive.
Consequently, there exists a need for a traceable Standard to assess multi-axis Coordinate Measuring Machines and, more specifically, the multi-axis, volumetric capability of the cooling holes patterns of turbine airfoils.